1. Field of the Invention
The present invention relates generally to wireless networks, and specifically to enabling seamless mobility for a multiple interface device in a co-located wireless environment.
2. Background of the Invention
Wide area cellular networks are being upgraded to support the Internet Protocol (IP). This will allow people to surf the web, check e-mail and access the Internet anywhere they could previously make a cellular phone call. These new wide area wireless systems are generally called third generation, or 3G, systems. They will offer bandwidths comparable to a few dial-up modem connections (usually up to 144 Kbps) to practically anywhere.
Coincident with this deployment of wide-area wireless networks is an increased deployment of so-called wireless local area networks (WLANs). These WLANs service areas on the order of 100 meters, use unlicensed spectrum, and offer top speeds on the order of 4-5 Mbps. Many WLAN systems are being deployed in shopping malls, coffeehouses, theaters, bookstores, public parks, etc. In the future these two systems, 3G and WLAN, will often service the same coverage area. This situation leads to what is called a co-located wireless environment. In such an environment a properly equipped mobile device is capable of, and has the option to, access the 3G network as well as the WLAN network.
More and more devices are being equipped with multiple interfaces, or the capability of supporting multiple interfaces. Almost all laptop devices today come with two PCMCIA slots that allow network interface cards to be plugged into the computer. Similarly many palm-top and Personal Data Assistant (PDA) devices come equipped with PCMCIA slots or Compact-Flash (CF) slots that allow for the same plug-in functionality.
Additionally many laptop and mobile device manufacturers are building embedded interfaces into their products. This means that a laptop comes pre-equipped with network access capabilities. This class of devices will be referred to as Multiple Interface Devices (MIDs). A MID is a device having the ability to communicate over more than one access network at the same time. One example of a MID is a laptop computer having an IEEE 802.11 PCMCIA card and a GSM or CDMA PCMCIA card. Another example of a MID is a cellular telephone having a CDMA radio and an embedded Bluetooth radio. Still another example of a MID is a PDA having a cellular radio attachment and an IEEE 802.11 interface.
Seamless mobility refers to an ability of a mobile device to change its point of attachment to a network (i.e., handoff) while not causing any noticeable disruption in service quality or disabling any active communication sessions. When mobile devices use IP protocols to communicate they are identified by a unique IP address. Traditionally the IP protocols where developed to handle fixed networks where there was no mobility. As such each IP address not only represents a mobile terminal's identity on the network, but also its location. This linking of identity with location causes problems when a mobile terminal is moved.
When a mobile terminal moves from one point of attachment to another point of attachment that is in a different subnet, it must acquire a new IP address in order to communicate. However, when it gets a new IP address it can longer receive information that was addressed to its old IP address. Consequently any on-going active sessions with the mobile terminal are disrupted and broken.
The industry has developed a suite of protocols, called Mobile IP, that address this mobility problem. See, for example, C. Perkins, Editor, “IP Mobility Support,” Request for Comments: 2002, Network Working Group, IBM Corporation, October 1996. Standard Mobile IP allows a mobile terminal to change points of attachment and continue to receive packets from on-going sessions, although there is small period of time when the mobile terminal is being configured and is unable to receive packets. There have been a number of industry efforts to reduce this configuration time, and consequently improve the user experience by making it more seamless, in what are called fast-handoff procedures.
More specifically, Mobile IP protocols assign a unique permanent Home Address to a mobile terminal. The mobile terminal will always use this address in all of its communications. Mobile IP also specifies mobility agents that exist in both the home network and in visited networks that assist in the mobility procedure. The Home Agent (HA) exists in the home network and manages all the traffic for the mobile terminal. An optional Foreign Agent (FA) exists in visited networks. When a mobile terminal moves from its home network to a visited network, it receives a new IP address, called a care-of address. Within the FA-mode, the mobile terminal will inform the FA of this address, its home address, and the address of its HA. The FA will then forward a Registration Request to the HA. The HA will accept this request and store a mobility binding that links that mobile's home address with the newly acquired care-of address. All traffic destined to the mobile terminal will be addressed to the mobile terminal's home address. The HA will intercept this traffic, consult the mobility binding for the mobile terminal, determine the proper care-of address associated with the mobile terminal's home address, and forward the packet to that care-of address. The Home Agent acts as an administrative assistant that forwards packets to the mobile terminal's new location. Anyone communicating with the mobile terminal is completely unaware of the fact that the mobile terminal has moved.
Mobile devices that have only one interface will experience a “black-out” period when they are unable to receive information. This black-out period lasts from the time that the mobile device acquires its new care-of address to the time that the HA registers that care-of address in its mobility binding.